%% http://lsr.di.unimi.it/LSR/Item?id=564

#(define-public (bracket-stencils grob)
  (let ((lp (grob-interpret-markup grob (markup #:fontsize 3.5 #:translate (cons -0.3 -0.5) "[")))
        (rp (grob-interpret-markup grob (markup #:fontsize 3.5 #:translate (cons -0.3 -0.5) "]"))))
    (list lp rp)))

bracketify = #(define-music-function (arg) (ly:music?)
   (_i "Tag @var{arg} to be parenthesized.")
#{
  \tweak Parentheses.stencils #bracket-stencils
    \parenthesize $arg
#})

\relative c'' {
  \bracketify c4
  \bracketify g4
  <e \bracketify g b>
  \bracketify <e g b>4
  \bracketify r4
}